1. Field of the Invention
The invention relates to a device for exposure metering in photographic cameras.
2. Description of Related Art
Photographic cameras with exposure metering through the image-recording objective are known. Conventional reflex cameras pivotally support a photoelectric receiver in front of the image plane for the exposure metering. The receiver has a limited recording area, so that only a portion of the image is utilized for metering. It is also known to provide a light-reflecting spot on the shutter curtain. The light reflected by the spot is converted into a metering signal by a photoelectric receiver arranged in the base of the camera housing. Integral metering over the entire image field of the exposure is not possible with conventional cameras.
DE 26 05 955 C2 discloses a reflex camera in which a reflector is provided with a grating structure which deflects imaging rays passing through the semitransparent reflex mirror toward a photoelectric receiver arranged in the camera base. The grating structure comprises a honeycomb mirror having a large number of convex or concave mirror elements of identical focal length. The mirror elements are inclined in such a way that the center of each individual element reflects the major rays toward the photoelectric receiver.
In addition, the reflector may have a slight curvature such that the marginal rays coming from the imaging objective are also reflected toward the photoelectric receiver.
The honeycomb mirror may be divided into a central mirror part which is used for spot metering and a larger part enclosing the central mirror part. The larger part enclosing the mirror part can optionally be connected to the smaller part for integral metering of the entire image field.
In the metering position, the reflector pivots into the imaging beam path of the camera. To take the picture, the reflector is folded out of the beam path together with the reflex mirror. Given the limited space available in the camera housing, the reflector must be configured as flat as possible, in order to be moved out of the imaging beam path.
The honeycomb elements scatter light, in an optimally diffuse manner, in the direction of the photoelectric receiver. The luminance of the area is proportional to the luminance of the object to be photographed, and proportional to the objective opening. Conventionally, single-part reflectors are used, and dividing the mirror into separate regions provides spot and integral metering.
A disadvantage of diffusely scattering the light with the reflector is a relatively small current is generated from the photoelectric receiver(s). Complicated electronics are necessary to amplify the current for the purpose of evaluating the measured signal.
DE 12 09 419 discloses a reflex camera in which a small-area photodiode is provided on a movable protective light flap in front of the shutter. The photodiode rests in a depression in the protective light flap and, for the purpose of metering, must be pivoted out of the depression into a position perpendicular to the imaging beam path. The arrangement of the photodiode directly behind the semitransparent reflex mirror generates acceptable photocurrents.
However, the depression interferes with the imaging beam path when the protective light flap is pivoted toward the reflex mirror during an exposure. In addition, the spot-like metering photodiode hampers fitting a metering system for the surrounding field, hence integral metering is not possible.
In addition to predefined constant object illumination, flash illumination is often provided. For a correct exposure, it is advantageous to measure the quantity of light directed by the flash onto the object before the exposure. Conventionally, this metering also occurs through the image-recording objective. Given the increased light provided for a brief period, a photodiode for spot metering can be used as a photoelectric convertor.
DE 31 49 655 C2 discloses a single circuit for metering both flash illumination and constant surrounding field illumination. The evaluation circuit can be switched such that, for flash illumination, the signal coming from the photodiode is integrated directly in the linear amplification range, and when metering the surrounding light, the signal is firstly logarithmically compressed.